1. Field of the Invention
The present invention relates to a thin-film transistor device manufacturing method, a thin-film transistor, and a display device.
2. Description of the Related Art
For example, a thin-film transistor (TFT) is included in a liquid crystal panel or an organic EL panel. A channel portion of the thin-film transistor is made of amorphous silicon a-Si, or polycrystalline silicon poly-Si which is a crystalline material. A crystalline silicon layer (poly-Si layer) of the channel portion of the thin-film transistor is typically produced by forming an amorphous silicon layer (a-Si layer) and then irradiating the amorphous silicon layer with laser light of an excimer laser or the like so that the amorphous silicon layer is instantly increased in temperature and crystallized.
There are two types of thin-film transistor structures, namely, a bottom-gate structure in which a gate metal is located on a substrate side as seen from x-Si (x is a or poly) of the channel portion, and a top-gate structure in which a gate metal and a source-drain metal are located on a side opposite to the substrate side as seen from x-Si of the channel portion. The bottom-gate structure is mainly used for a a-Si TFT whose channel portion is formed of an amorphous silicon layer, whereas the top-gate structure is mainly used for a poly-Si TFT whose channel portion is formed of a crystalline silicon layer. The bottom-gate structure is commonly used as a structure of thin-film transistors in a liquid crystal panel or an organic EL panel used for a large-area display device.
There is also an instance where the poly-Si TFT has the bottom-gate structure, which provides an advantage of a reduction in manufacturing cost. In the poly-Si TFT having the bottom-gate structure, the crystalline silicon layer is formed by irradiating an amorphous silicon layer with laser light to crystallize the amorphous silicon layer. In this method (laser annealing crystallization), the amorphous silicon layer is crystallized by heat generated by laser light irradiation.
Thin-film transistors in an organic EL panel are particularly required to have uniform characteristics. Applying the aforementioned laser annealing crystallization to manufacturing of the thin-film transistor of the bottom-gate structure, however, has the following drawback (problem). In the thin-film transistor of the bottom-gate structure, first a gate electrode is formed using a metal material of a higher heat conductivity than silicon or an insulation film, and then an insulation layer and an amorphous silicon layer are formed. Accordingly, when irradiating the amorphous silicon layer in the bottom-gate structure with laser light to crystallize the amorphous silicon layer by laser annealing crystallization, heat that is supposed to be used for crystallizing the amorphous silicon layer is absorbed and transmitted by the gate electrode, making it impossible to sufficiently crystallize the amorphous silicon layer. This causes decreased crystallinity or non-uniform crystallinity.
In view of such a problem, there is disclosed a method of disposing a dummy gate pattern in a nearby region of the gate electrode, i.e. a channel neighborhood, to reduce a difference in heat capacity between the amorphous silicon layer located above the gate electrode and the amorphous silicon layer located above the dummy gate pattern (for example, Patent Literature 1: Japanese Unexamined Patent Application Publication No. 10-242052). There is also disclosed a method of extending the gate electrode to a laser light scan upstream side so that, though the use of a pre-annealing effect of the extended portion of the gate electrode, the gate electrode is thermally saturated before laser light reaches the channel region of the thin-film transistor, thereby keeping the gate electrode from absorbing heat generated in the silicon thin film (for example, Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2007-035964).